The present invention pertains to high speed laser scanning equipment and processes for using it in the high speed laser scribing of ferromagnetic sheet material to refine magnetic domain spacing.
The development of high permeability grain oriented silicon steel resulted in a significant reduction in core loss, especially at inductions greater than 1.5 T (15 KG). This reduction in loss has been achieved primarily by improvements in the degree of grain orientation. Separation of the components contributing to the overall core loss has shown that the improved losses obtained are due to a reduction in the hysteresis component of the core loss. Further loss reduction can be achieved by refining the 180.degree. domain wall spacing, which results in a lowering of the eddy current component of core loss.
Over the past several years techniques have been developed to reduce the domain wall spacing by changing the magnetostatic or the magnetoelastic energy in the sheet. Insulative coatings that apply a tensile stress parallel to the rolling direction have been effective in reducing the domain wall spacing and the core loss. Mechanical, or physical, scribing transverse to the sheet rolling direction is another technique that has been found to be effective in reducing domain spacing and lowering the losses. The disadvantages of mechanical scribing are that the insulative coating is disturbed, and the space factor is decreased.
Efforts to obtain the advantages of scribing without the aforementioned disadvantages have centered around the use of pulsed laser scribing techniques. It is known that irradiation of an iron-silicon alloy by a laser pulse of sufficient power density can vaporize material at the alloy surface causing a pressure shock wave to travel through the alloy causing dislocations and twins (see A. H. Clauer et al, "Pulsed Laser Induced Deformation in an Fe-3 Wt. Pct Si Alloy," Metallurgical Transactions A, Vol. 8A, January 1977 pp. 119-125). This deformation, like the deformation produced by mechanical scribing, can be used to control domain spacing. In fact, pulsed lasers have been applied to grain oriented electromagnetic steel sheet to produce shock wave induced arrays of deformation (see, for example, U.S. Pat. No. 4,293,350, French Patent Application No. 80/22231 published on Apr. 30, 1981 Publication No. 2,468,191, and European Patent Application No. 0033878 A2).
Despite the above efforts, there still exists a need for the development of instrumentation and processes capable of scribing at the high speeds necessary to make scribing of large lots of heats or ferromagnetic material practical on a high volume high throughput basis. The present invention addresses these needs.
Applicants have developed machines and processes that are capable of meeting these needs. According to the present invention this equipment includes a rotatable means for deflecting a laser beam and a means for focusing the laser beam onto the object to be scribed. Also included in this machine is a means for moving the object to be treated through the path of the laser beam. When said rotatable means for deflecting the laser beam is rotating it translates the laser beam substantially transverse (i.e. within .+-.45.degree. of perpendicular to) to the direction of movement of the object.
In one embodiment of the present invention the means for focusing and means for deflecting may be incorporated into one device, however, in other embodiments, where they are separate entities, it is preferred that a means for focusing be positioned between the means for deflecting the beam and the sheet of material to be treated.
The rotatable means for deflecting is preferably a mirror coupled to a variable speed motor.
The focusing means may be a lens or a mirror. Preferably it has a cylindrical type geometry so as to produce an elongate beam spot on the sheet being scribed.
Included within the present invention is a laser beam source, preferably one capable of operating in a continuous wave or extended pulse mode, such as a CO.sub.2, Neodymium YAG or Neodymium Glass laser.
The means for translating the sheet past the laser preferably includes rolls, or a table having a vacuum check or a magnetic chuck, suitably shaped to elastically curve the sheet and thereby keep the sheet surface being scribed within a predetermined distance of the focal plane of the focusing means across the entire path of the laser. Preferably the sheet surface is maintained, as nearly as possible, at the focal plane of the focusing means.
Alternatively, also in accordance with the present invention, the sheet surface may be held in a planar configuration during scribing. In this embodiment according to the present invention the rotatable means for deflecting includes groups of mirrors circumferentially as well as axially distributed around a common axis of rotation. This common axis of rotation is aligned substantially transverse to the direction of movement of the sheet (i.e. rolling direction) and the mirrors within each group are orderly distributed over a length, parallel to the axis, that is substantially equivalent to the width of the sheet. Upon rotation each mirror is sequentially brought into the path of the laser beam deflecting it onto the sheet surface and incrementally moving the beam across the sheet surface.
Also in accordance with the present invention, a process for using the aforementioned laser scribing instruments is disclosed. A ferromagnetic sheet traveling in a first direction at a speed, S.sub.1, is scribed by a laser beam traveling in a second direction substantially transverse to said first direction, and at a speed, S.sub.2. The laser beam produces a predetermined beam spot size and shape on the sheet and scribe lines having a predetermined spacing. In this process S.sub.2 and P, the incident power of the beam, are selected in such a manner that the following conditions are satisfied:
(1) 0.1.ltorsim.PS.sub.2.sup.-1/2 .ltoreq.7 (where P is in watts and S in inches/minute). PA1 (2) The laser beam power density does not produce shock deformation. PA1 (3) The laser beam incident energy density is greater than 10 and less than about 200 joules/cm.sup.2.